Millimeter-Wave Induced Heating of Cutaneous Nerves and Capillaries

In this study, we quantify microscale heating at the level of cutaneous nerves and capillaries due to continuous and pulsed plane-wave exposure at 60 GHz. The thermal properties of the nerves and capillaries were derived using mixture equations based on their water content. The electromagnetic problem was solved in conjunction with Pennes bioheat equation and Arrhenius equation using finite element method to evaluate the spatial and temporal evolution of temperature along with thermal damage within cutaneous nerves and capillaries. Although, the maximum power density within the nerve (41.6 kW/m ³ ) and capillary (20 kW/m ³ ) was 37.3% and 30.2% higher than surrounding skin for a continuous exposure at 10 W/m ² , the peak temperature elevation ( $\Delta T$ ) within the nerve (93.3 n°C) and capillary (90.7 n°C) occurred after 5 $\mu$ s and 10 $\mu$ s of exposure and was 19.2% and 17.7% higher than surrounding skin, respectively. The nerve and capillary attained thermal equilibrium with skin after roughly 10 ms. The maximal $\Delta T$ within the nerve (0.5 °C) and capillary (0.25 °C) due to nano- and micro-second 60 GHz pulses with highest fluence (0.48 kJ/m ² ) permitted under ICNIRP guidelines was 34% and 24% higher than in the surrounding skin. Ten 3 μs 60 GHz pulses (power density = 13.4 GW/m ² ) separated by 10 s of cooling period were used to demonstrate the possibility of selective thermal ablation of cutaneous nerves [damage index ( $\mathit {\Omega }$ )=1.1)] without damaging skin ( $\mathit {\Omega }$ = 0.15). The results provide valuable insights into local millimeter-wave induced heating within various skin substructures.